Harmonics and VFDs

The input stage of a Variable Frequency
Drive is a rectifier that converts the AC supply voltage into a
DC voltage for the DC Bus. The output stage chops up the DC and
converts it back to AC.

The action of the rectifier distorts
the input current which is no longer sinusoidal.
A distorted waveform can be analysed as the fundimental frequency
and a series of harmonics. In essence, any distorted waveform can
synthesized by the addition of the fundemental sinewave and harmonic
sinewaves of varying amplitude and phase angle. In the case of a
VFD, the input rectifier is a full wave rectifier and so the harmonics
are Odd order only.

Q. What is a harmonic?
A. A Harmonic is a signal at a frequency that is a multiple of the
fundimental or base frequency.
Odd mulitples (3rd, 5th, 7th,....) are known as odd order harmonics and
are caused by symmetrical distortion or the waveform.
Even multiples (2nd, 4th, 6th,...) are known as even order harmonics and
are caused be assymetric distortion. - the positive and negative halves
of the waveform are distorted unequally.
Half wave rectifiers cause even order harmonics and full wave rectifiers
cause even order harmonics.

Q. What problems are caused by harmonics?
A. Harmonics cause issues with other connected equipment, especially
capacitive loads and rotating machines. If a distorted voltage waveform
is conected to capacitors such as power factor correction capacitors,
the current flowing will increase and cause higher losses in the capacitor.
The impedance of a capacitor is inversely proportional to the frequency.
If we apply line frequency to the capacitor, there will be a current flow
through the capacitor of X Amps. If we apply an equal voltage across the
capacitor with a frequency of 5 times line frequency, then the current
through the capacitor will be five times higher than at line frequency.
If the harmonic voltage is 10% line voltage at the fifth harmonic, then
the current at the fifth harmonic is half of the line current. The addition
of harmonics to the line voltage can cause a significant increase in the
current through the capacitor and result in early failure.
In the case of an induction motor, the presence of harmonics causes torque
fields running at the frequency of the harmonic. When the motor is operating
at full speed, there is high slip at the harmonic frequencies. This causes
high pwer dissipation in the rotors and with high resistance motors, (as
found in submersible motors), can result in early rotor failure.
High harmonic current increase the I2R losses and cause in
increase in supply losses. Transformers, fuses, cables etc must be oversized
to allow for the increased heating losses caused by harmonics.

VFD Designs

In low cost VFDs, the input rectifier is directly connected
to a large DC Bus capacitor bank. With this configuration, the charging
current only flows while the instantaneous supply voltage is higher than
the DC bus voltage. This means that the current only flows during the
crest of the voltage waveform, a very high curent for a very short time.
This causes high order harmonics of significant amplitude.
Better VFDs include either DC Bus chokes or AC line reactors to reduce
the amplitude of the peak currents. The AC reactors or DC bus chokes cause
the current waveform to change from a very high current for a few degrees,
to a lower current for longer. The maximum conduction angle with a three
phase rectifier, is 120 degrees.
The effect on the harmonics, is that the high amplitude current for a
short time has significant harmonics to a very high order. As the conduction
angle is lengthened, the order of the harmonics is reduced and the Total
Harmonic Distortion is also reduced.
The harmonics produced by a three phase input VFD are primarily the 5th,
7th, 11th and 13th. The triplen harmonics (3rd, 9th, 15th) are not generated
by a correctly operating three phase rectifier.
The THDi of a VFD without a DC Bus choke or AC line reactors is in the
order of 81% and the THDi of a VFD with a DC bus choke or AC line reactors
is in the order of 35%.
As the load is reduced, the THDi increases.